As I understand it, what you are seeing with the light is a backing that is being put on the back of the swatches. This backing is then adhered to the card as well, which means if the swatch is removed, the backing is damaged and unreadable/ no longer there. Kind of like those stickers they have on electronics, that when you open the package the sticker says VOID VOID VOID all over the spot it was removed from.

It should be noted that is a somewhat recent thing, don't remember when it was started. You could still have older legit swatches without this.

''I've never been in style, so I can never go out of style." - Lillian Gish ( 1
893 - 1993)